Make RegisterDial safe to call from multiple goroutines. (#773)

Fixes #772

driver.go

@@ -20,6 +20,7 @@ import (
 	"database/sql"
 	"database/sql/driver"
 	"net"
+	"sync"
 )
 
 // watcher interface is used for context support (From Go 1.8)
@@ -35,12 +36,17 @@ type MySQLDriver struct{}
 // Custom dial functions must be registered with RegisterDial
 type DialFunc func(addr string) (net.Conn, error)
 
-var dials map[string]DialFunc
+var (
+	dialsLock sync.RWMutex
+	dials     map[string]DialFunc
+)
 
 // RegisterDial registers a custom dial function. It can then be used by the
 // network address mynet(addr), where mynet is the registered new network.
 // addr is passed as a parameter to the dial function.
 func RegisterDial(net string, dial DialFunc) {
+	dialsLock.Lock()
+	defer dialsLock.Unlock()
 	if dials == nil {
 		dials = make(map[string]DialFunc)
 	}
@@ -66,7 +72,10 @@ func (d MySQLDriver) Open(dsn string) (driver.Conn, error) {
 	mc.parseTime = mc.cfg.ParseTime
 
 	// Connect to Server
-	if dial, ok := dials[mc.cfg.Net]; ok {
+	dialsLock.RLock()
+	dial, ok := dials[mc.cfg.Net]
+	dialsLock.RUnlock()
+	if ok {
 		mc.netConn, err = dial(mc.cfg.Addr)
 	} else {
 		nd := net.Dialer{Timeout: mc.cfg.Timeout}